An AlxGa1-xN group XIX nitride semiconductor, having a high Al composition has higher band gap energy than GaN or SiC, which is generally known as a wide band gap semiconductor, and therefore is expected to be applied to various types of semiconductor devices, such as deep ultraviolet light-emitting elements and electronic devices having higher withstand voltage characteristics than ever before.
In producing such a semiconductor device, a technique rot obtaining favorable ohmic characteristics, by reducing a contact resistance between an AlxGa1-xN semiconductor layer having a high Al composition and an electrode, becomes very important from the viewpoint of reducing a power loss in the device and joule heat produced owing to the power loss. However, since an increase in the Al composition makes it difficult to obtain the favorable ohmic characteristics, a lot of studies have been performed on art electrode material, a formation method thereof, and the structure of a contact layer having an n-type AlxGa1-xN semiconductor layer, as a method for forming an ohmic electrode on the n-type AlxGa1-xN semiconductor layer having a high Al composition. For example, Patent Literature 1 proposes a method in which a first, electrode metal made of Ti, V, and Ta is formed on an n-type AlGaN layer and then a heat treatment is applied thereto at a predetermined temperature. Furthermore, a second electrode metal including a high conductive metal is formed on the first electrode metal. As other methods. Patent Literatures 2 and 3 propose structures to reduce a contact resistance by forming an n-type AlGaN layer, having a low Al composition as a contact layer on a surface on which electrodes are to be formed.